The present invention relates to a thermostat valve that utilizes the reversible expansion and contraction of a thermally expandable material, as typically used in internal combustion engines.
Bimetal and bellows type thermostats were originally employed in low pressure/low temperature applications. With the growing use of pressurized cooling systems, designed to raise the boiling point of the cooling fluid, thermostats employing an "incompressible" thermally responsive material, such as wax or a wax--metal mixture, have come into wider use.
The mechanics of the conventional wax-pellet thermostat are well known, as illustrated by U.S. Pat. No. 3,591,075 by Oaishi. The thermostat is placed in the coolant outlet, so that the wax containing reservoir rests in the engine block coolant. When the engine is cool a spring ensures that the valve member maintains contact with the valve seat, blocking coolant flow from the engine to the cooling reservoir (typically the cooling radiator for land vehicles or the body of water for marine use). As the engine warms, the wax within a soft elastic bladder is heated and expands. This produces a force against a piston in contact with the valve member. When the coolant temperature reaches a predetermined level, the force of the bladder on the piston overcomes the spring force joining the valve member and valve seat. The valve member and valve seal separate, allowing cooling fluid to flow. As the temperature continues to rise, the wax continues to expand, and the valve continues to open until steady state is reached or the piston full stroke is reached.
Shortcomings of this thermostat are well documented. Oftentimes the container for the wax material leaks or otherwise fails. This causes the thermostat to remain closed, eventually resulting in engine overheating. The piston is also subject to sticking or hanging up, usually in the closed or fully open position. The former will result in an overheated engine, while the latter will cause poor engine performance, excessive exhaust, and again overheating if the increased heat transfer from the radiator to the environment is inadequate, thus resulting in heat build up in the reservoir.
Numerous improvements to the basic design exist that improve the functionality of the thermostat. One improvement employs a casing member, containing a thermally responsive material acting on a piston member which slides through a cylindrical guide, that is either machined with tight clearances or employs a seal to allow the piston to repeatedly axially translate from the closed to open position. However, this improvement involves more parts, which increases both the probability of any one part failing and the cost of manufacturing.
Another drawback to this design is that it promotes piston sticking or bladder wear, since one end terminates at the wax bladder, while the other end is open to the cooling fluid forming a "dead leg", a place where contaminants and debris tend to accumulate. Contaminants can also enter the pellet area causing bladder wear, particularly in cold temperatures.
Most of the present art employs a separate spring member to ensure that the thermostat is biased in the closed position. This usually results in an engine overheat upon failure. This design also suffers the same shortcomings regarding the additional parts involved, namely the increased probability of the failure of any single component and the increased manufacturing cost.
Several alternatives to alleviate this shortcoming have been developed, such as U.S. Pat. No. 5,381,952 by Duprez. Duprez reveals a thermostat designed to fail in the opening position. This design reveals the basic wax thermostat design discussed above with an added spring to force the thermostat into an open position upon failure of the thermostat. This design employs added complexity and parts, both which adversely affect the probability single component failure and added manufacturing cost.
Other solutions have been employed, such as redundant thermostat valves or manufacturing the thermostat incorporating fusible elements. However, both these solutions adversely impact manufacturing cost.
Additionally, the redundant valve is typically sized to provide only enough coolant flow to allow the engine to operate under no-load conditions. Thus, the net effect is to save the engine, but strand the operator with an engine that cannot perform.
The latter method, employing fusible members in the construction of the thermostat, typically requires exotic materials and is slow to react. Other factors besides temperature also promote failure, such as cycle fatigue. In the event that the fusible member works as designed, the valve fails completely open, with all the cooling fluid going through the cooling reservoir with the same results of a stuck open thermostat described previously.
The present invention solves the problems present in the field. First, the invention provides a reliable thermostat with fewer parts. Further, the present invention minimizes the number of moving parts. Thirdly, the invention provides a reliable thermostat without a fluid "dead leg" where contaminants and debris tend to accumulate. In addition, the present invention fails in the open position, and can be designed to fail open in a preset prescribed position. Lastly, the invention is a simple design and can be easily fabricated or assembled, thus saving manufacturing costs.
The invention consists of a reservoir containing a thermally expandable material. The base of the reservoir is a resilient flexible plate. A stem connects the resilient flexible plate to a valve seal which acts as a closure for a valve port.
In the cooled state, the expandable material occupies the whole of the volume of the reservoir. In the unheated state, the plate is biased concavely inward towards the reservoir volume. Thus, in the unheated state, the valve member is seated against the valve seal and blocking the valve port.
When heated, the material expands and exerts pressure against the resilient plate. This force operates to bend the plate away from the reservoir and drive the stem outwards. This serves to free the valve seal from the valve opening and thus allowing coolant to circulate through the valve opening.
If the reservoir should develop a leak, a neutral pressure would occur in the reservoir relative to the operating environment. Thus, the plate would naturally straighten, forcing the stem outwards. The valve would then be placed in a predetermined open position, allowing a predictable flow of coolant through the opening when being operated.